A variety of vehicles, such as automobiles and aircraft, carry electronic equipment which requires an antenna. Historically, antennas for vehicles consisted of structures which protruded from and were mounted to the outer surface of the body or skin of the vehicle. An example is the common automobile whip antenna. The prior art then came to recognize that such antennas created aerodynamic problems, were easily broken, were not aesthetically appealing, and required holes through the outer body or skin, thus interrupting the integrity of that outer body or skin of the vehicle.
Prior art workers have attempted to reduce these problems by constructing antennas which conform to the shape of the outer surface of the vehicle. For example, an antenna has been formed in an automobile windshield by embedding a pair of inverted L-shaped wires in between the two glass layers of the windshield. Such antennas consisted of a pair of parallel spaced wires extending upwardly from the bottom center of the windshield which then diverged in opposite directions along but spaced from the top of the windshield. The problem with these antennas is that they exhibited undesired directional patterns and were visible to the operator of the vehicle and consequently were aesthetically undesirable.
Another proposed solution was the formation of a conventional slot antenna within the car body, but concealed behind an electromagnetically transparent material, such as a synthetic resin. Such antennas are illustrated in U.S. Pat. No. 4,707,700 and SAE Technical Paper entitled "A New Generation Of Antennas For Automobile Use" by Louis L. Nagy, reprinted from SP-701, Recent Developments In Automotive Audio Systems, 1987.
Yet another approach was shown in U.S. Pat. Nos. 2,971,191 and 3,007,164 which utilized the entire window opening as an aperture antenna.
Thin metal films are currently used in windshields as a solar shield to filter or reflect infrared and ultraviolet radiation in order to minimize the solar heating of the cat's interior. They may also be attached to DC sources so that an electrical current resistively heats the film to defrost the window. A substantial body of prior art exists teaching the application of such films to windows and a substantial body teaching resistive heating. Representative patents include: U.S. Pat. Nos. 4,610,771; 4,782,216; 4,806,220; 4,898,790; and 5,012,255.
Several antennas have been proposed which use the concepts of a whip antenna or a dipole antenna and combine it with the concept of forming such an antenna in a vehicle window by thin films on or between the layers of the window. Antennas of a variety of shapes are formed on the surface of the windows and often are provided with auxiliary or supplemental antennas in an attempt to control the antenna pattern or gain. Diversity selection has previously been used for antennas.
For example, U.S. Pat. Nos. 4,768,037 and 4,849,766 illustrate a variety of different antenna shapes formed by a thin film upon a layer of a vehicle window. U.S. Pat. No. 4,864,316 and Japanese Kokai patent Hei 2[1990]-113603 illustrate other shapes formed in the window. Japanese Kokai patent Hei 2[1990]-15701 shows an antenna on a rear window. U.S. Pat. No. 5,005,020 shows a film antenna with a surrounding bus bar. Further improvements to these antennas are needed to obtain the performance necessary to gain broad commercial acceptance of antennas formed integrally with a window.
The principal problem in vehicle mounted antenna design is to obtain an antenna which not only has low loss and desirable impedance characteristics but sometimes also, very importantly, an azimuthally omnidirectional pattern so that it can receive or radiate a strong signal which is substantially insensitive to its direction with respect to the vehicle so that the signal strength will not vary appreciably as the vehicle is turned.